core: replaced calls to client.xmlstream.send by client.send which is the right method to use. client.xmlstream should not be used directly

src/plugins/plugin_sec_otr.py

@@ -94,7 +94,7 @@ class Context(potr.context.Context):
                      'timestamp': time.time(),
                     }
         self.host.generateMessageXML(mess_data)
-        client.xmlstream.send(mess_data['xml'])
+        client.send(mess_data['xml'])
 
     def setState(self, state):
         client = self.user.client